Advances in polymerization and catalysts have produced new polymer resins having improved physical and mechanical properties useful in a wide variety of products and applications. With the development of new catalysts, the choice of polymerization, such as solution, slurry, high pressure or gas phase, for producing a particular polymer has been greatly expanded. Advances in polymerization technology have also provided more efficient, highly productive, and economically enhanced processes.
Polymerization catalysts may be combined with an activator to yield improved catalyst compositions. Methylaluminoxane (“MAO”) is a common activator for catalysts that produce polyethylene and other polyolefins. While MAO can provide improved catalyst functionality, the MAO is typically a major component of catalyst cost. Approximately half of the total cost of the MAO can be attributed to its trimethylaluminum (“TMA”) precursor. Additionally, as MAO is pyrophoric and reactive with any chemical bearing an acidic proton, it may require special storage, handling, and transport measures. Thus, cost savings could be achieved by processes that more efficiently use the TMA precursor or minimize the amount of storage, handling, and transport needed for the MAO.
A common process for producing MAO utilizes a controlled hydrolysis of a dilute TMA solution to form MAO and methane. The hydrolysis reaction is typically carried out in a solvent, such as toluene, which can increase the efficiency of the hydrolysis reaction. While helping to absorb the heat of the TMA hydrolysis, the excess toluene may also be needed to keep the MAO in solution and ensure that water concentrations remain low. The MAO production process may also include refining by means of a flash evaporation to recover some of the toluene and unreacted TMA while concentrating the MAO from less than 10 weight percent up to approximately 30 weight percent in toluene for shipment. Although shipping the MAO in this concentrated MAO solution is common, there are drawbacks to this MAO production process. For example, the concentrated MAO solution still contains unreacted TMA that may not be consumed in the subsequent catalyst manufacturing operation, which must ultimately be disposed. In addition, significant quantities of toluene from the concentrated MAO solution are also passed through the catalyst production process as a waste stream. The concentrated MAO solution may also have a limited shelf life before coagulating into an unusable gel, even when kept in cold storage. In some instances, stabilizers may be added at an additional expense to create a modified MAO with improved shelf life.